What a shorted bypass diode looks like

Crystalline-silicon modules are built from two or three series-connected substrings, each shunted by its own bypass diode. The diode's job is to turn on and route current around a substring when that substring is shaded or otherwise can't carry the string current — a normal, protective event. A shorted bypass diode is different: the diode has failed closed, so its substring is bypassed permanently, shading or not, and stays bypassed under STC too.

Why the signature is a step, not a slope

A shunt-resistance problem (see the PID case study) degrades continuously — R_sh can be anywhere from "slightly low" to "very low," so the curve droops by a varying amount. A shorted bypass diode is binary at the substring level: either that diode is conducting (substring fully bypassed) or it isn't. For a module built from 2 or 3 substrings, that means Voc can only land on one of a small number of discrete levels — 100%, roughly 2/3, or roughly 1/3 of the healthy Voc for a 3-substring module — never smoothly in between.

Modeled IV curve overlay: healthy module in green vs. the same module with one of three substrings bypassed, in red. The red curve keeps nearly the same current level but cuts off at about two-thirds the healthy Voc. — Modeled illustration — one of three substrings' cells removed from the voltage-generating path; current-carrying capability is largely unaffected.

Notice what stays the same: the current level near the knee is close to the healthy curve's — the remaining two-thirds of the cells still generate current normally, and the bypass diode routes it around the dead substring rather than blocking it. What changes is almost entirely Voc, because one substring's cells no longer contribute their voltage to the sum.

The Voc-quantization feature

PV Ivy's diagnosis engine tracks voc_deficit — one minus the ratio of measured to modeled Voc — specifically looking for values that cluster near fractions like 1/3 or 2/3 rather than a continuous range. That's the practical difference between "this module's Voc is a little low, investigate broadly" and "this looks like exactly one bypass diode has failed shorted."

Bar chart of Voc for a three-substring module at 0, 1, or 2 substrings shorted: 37.9V, 25.3V, and 12.6V respectively — discrete steps, not a continuum. — Modeled illustration — the same module's Voc at each possible bypass-diode failure count. Real modules will vary substring cell-counts slightly, so expect these fractions approximately rather than exactly.

A continuously variable Voc loss (say, 40–95% of reference, anywhere in between) points toward soiling, partial shading without a functioning bypass path, or connector/wiring resistance — not a shorted diode. A Voc ratio sitting close to one of the quantized levels for that module's substring count is the actionable signature.

String-level implications

A shorted bypass diode doesn't just cost that one module's power — it changes the string's Vmp at a fixed current, which matters for inverter MPPT windows: a string with one module losing a third of its Voc contribution needs the inverter to track a lower window than the as-designed string Voc(Vmp) range assumed. On a long string already sized close to an inverter's minimum MPPT voltage, one shorted diode can be the difference between the string tracking normally and the inverter clipping or falling out of its MPPT range on a hot day. This is exactly what the Analyzer's MPPT-limiting check (V_mppt,min = max(Mppt_low, Vac·√2)) is for — it flags the projected power impact, not just "Voc is low."

Reproduce it

Paste a suspect module's field or flash-test V/I curve into the IV Curve Explorer's Add measured data and compare Voc against the modeled reference curve directly.
Run the same curve through the IV Data Analyzer for the full validation + STC translation + voc_deficit check, and to see whether the string's Vmp still clears your inverter's minimum MPPT voltage under worst-case conditions.